Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as energy sources. Among these secondary batteries, lithium secondary batteries having high energy density and voltage, long lifespan and low self-discharge are commercially available and widely used.
Among components of lithium secondary batteries, a cathode active material has an important role in determining capacity and performance of batteries.
Lithium cobalt oxide (LiCoO2) having superior physical properties such as superior cycle characteristics is generally used as a cathode active material. However, cobalt used for LiCoO2 is a metal, so-called “rare metal”, which is low in deposits and are produced in limited areas, thus having an unstable supply. Also, LiCoO2 is disadvantageously expensive due to unstable supply of cobalt and increased demand of lithium secondary batteries.
Under these circumstances, research associated with cathode active materials that are capable of replacing LiCoO2 has been continuously made. Representative substitutes include lithium composite transition metal oxides containing two or more transition metals of nickel (Ni), manganese (Mn) and cobalt (Co).
The lithium composite transition metal oxide exhibits superior electrochemical properties through combination of high capacity of lithium nickel oxide (LiNiO2), thermal stability and low price of manganese in layered-structure lithium manganese oxide (LiMnO2), and stable electrochemical properties of LiCoO2, but is not easy to synthesize through a simple solid reaction.
Accordingly, the lithium composite transition metal oxide is prepared by separately preparing a composite transition metal precursor containing two or more transition metals of nickel (Ni), manganese (Mn), and cobalt (Co) using a sol-gel method, a hydrothermal method, spray pyrolysis, coprecipitation or the like, mixing the composite transition metal with a lithium precursor, followed by mixing and baking at a high temperature.
In terms of cost and production efficiency, a composite transition metal precursor is generally prepared by coprecipitation.
In conventional methods, composite transition metal precursors were prepared by coprecipitation, based on research that focuses on formation of spherical particles, such as optimization of particle size, in order to prepare lithium composite transition metal oxide that exhibits superior discharge capacity, lifespan, and rate characteristics when used as a cathode active material. Structure properties of composite transition metal precursors as well as formation of spherical particles thereof are considerably important.
However, a conventional coprecipitation reactor, for example, a continuous stirred tank reactor (CSTR) has a problem of low retention time taken for controlling the structure of composite transition metal precursors.
Also, due to long retention time, precursor particles prepared using a conventional coprecipitation reactor have a wide particle size distribution and a non-uniform particle shape and contain a great amount of impurities.
Also, in a case in which precursor particles are prepared using a conventional coprecipitation reactor, it is disadvantageously difficult to adjust the mean particle size of the precursor particles to a level smaller than 6 μm.